Although : is sufficient to indicate that the family should be INET6, it's not necessary. For example, one could specify localhost, which may resolve to 127.0.0.1 or ::1 or both. Or one may bind to '' or None, which will resolve to all interfaces. In that case, INET6 or AF_UNSPEC might be more appropriate.

Plus, I'd prefer that the default behavior should be to bind to IPv6, which in many cases also supports binding to IPv4 (at least for ::, all interfaces), and only fall back to the older IPv4 standard when necessary or directed. I do say though, with as few people familiar with IPv6, it's probably a hard sell to make it the default, even if that's eventually where the code should end up and it's been more than a decade in the transition.